Recent Federal Aviation regulations require that new transport aircraft include systems that provide for safe fuel tank operation. A flammable fuel mixture may occur as a function of the ullage portion of the fuel tank and environmental factors such as temperature and altitude.
Three conditions are needed for ignition: a fuel, an oxidizer, and an ignition source. In a fuel tank, fuel is always present, ignition source and oxidizer may also be present. The Federal Aviation Administration (FAA) is working to eliminate ignition source in commercial aircraft fuel tanks. The military is well aware that when aircraft are operating in battlefield environments, eliminating ignition source is impossible. Both FAA and the military recognize reducing oxidizer in fuel tanks to below ignition limit is the best way to avoid fuel tank ignition.
There were seventeen fuel tank ignition incidents since 1959. Three occurred since 1990 resulting in 239 fatalities. It had been typically assumed by the FAA that the cause for these incidents is flammable fuel vapors in ullage portion of fuel tank during flight. New Federal regulations require that fuel tank installation must include either a means to minimize the development of flammable vapors in fuel tanks or a means to mitigate the effects of an ignition of fuel vapors within fuel tanks. This requirement applies to all transport category aircraft.
During the Vietnam War, thousands of aircraft were lost due to enemy ground fire, from small arms fires to anti-aircraft artillery and surface-to-air missiles. Analysis indicated that fuel tank fire and explosion was the major cause of aircraft due to ballistic impacts. Fuel tanks on military aircraft must also be protected.
There are conventional systems that attempt to protect the fuel tank by minimizing the development of flammable vapors in the fuel tank. For example a flammability reduction system such as fuel tank inerting system is utilized. In another example an ignition mitigating device such as foam is utilized to eliminate fuel tank ignition probability. To describe these conventional systems in more detail, refer now to the following discussion.
1. On-board inert gas generation system (OBIGGS)—OBIGGS may be used to generate oxygen-depleted gas to inert the ullage space in fuel tanks. Inerting the ullage portion of the fuel tank reduces the oxidizer in the fuel tank and therefore reduces the flammability of the vapor therewithin.
2. Passive inert system—bottled gas, such as nitrogen or carbon dioxide, or carbon dioxide gas from dry ice, may be used for fuel tank inerting.
3. Polyurethane foam—foam may be installed in fuel tanks. It prevents an explosion by removing energy from the combustion process through absorption of heat and mechanical interference.
All of the above-identified systems have problems associated therewith. The following will describe some of the problems associated therewith.
Firstly, OBIGGS is a complex machine. It separates oxygen and nitrogen, the two main components of air. The oxygen is not used and nitrogen is pumped into fuel tanks to reduce the oxygen concentration in the ullage portion above the fuel in fuel tanks. When the ullage portion gas fuel/oxygen ratio is below the ignition limit, no spontaneous combustion will take place. In addition, OBIGGS is an expensive addition to an aircraft. For example in the Apr. 24, 2006 issue of Aviation Week & Space Technology it was reported that “about 3,814 transports in the U.S. fleet alone would be affected at $140-220 million per aircraft over a seven year period, according to the FAA” by using the OBIGGS system
Secondly, a passive system such as a bottled gas and dry ice would add significant logistics requirements to the aircraft. In addition the passive system would also require special personnel to handle the bottled gas and dry ice.
In some aircraft, foam may be used to deter ignition in fuel tanks. However the foam is heavy; does not allow total use of available fuel in the tank; and deteriorates over time. In addition replacing the foam in the fuel tank is messy and dangerous.
What is needed, therefore, is a system and method to overcome these issues. The present embodiment addresses such a need.